Various partial or complete replacements of the hip joint have been proposed and used since the early 1900's. Most of the procedures or methods involved material problems that led to joint loosening and consequential failure.
One widely used total hip replacement removes the femoral head and inserts a stem into the upper end of the femur, where it is fixed either by cement or by bone growth into a porous coating. A small metal ball, replacing the patient's femoral ball, is affixed to the stem. This technique requires massive bone removal and results in extreme loading of force in a leveraging action from the top of the femoral ball to a lower part of the stem. Over a few years, force applied by vigorous and mostly younger patients can cause the stem to loosen, resulting in failure, pain and need for extensive and expensive revisions.
When a patient has enough bone stock, a resurfacing approach may be used. In one example, the femoral head is resurfaced with a singular fixation approach that uses either cementing or bone ingrowth into a porous surface or surfacing material. However, with the singular fixation approach, the probability of failure remains high because there is no mechanism to back up, reinforce and absorb large downward forces and subsequent reactionary forces applied through vigorous activity.
Accordingly, after a hip joint replacement utilizing prior art techniques, slight movements (up to about 0.08 mm per year) of a replacement shell relative to a femoral ball may occur and are deemed “acceptable migration.” Single point fixation methods typically mitigate this problem with respect to only one degree of freedom in movement; for example, a centering post can stop a cap or shell from migrating longitudinally, but does not prevent it from migrating equatorially. The polar north direction is not a problem, in that anatomically most force applied to a femoral head is driving the shell on, versus pulling it off. Cement is capable of preventing loosening in the polar north direction. However, the equatorial direction receives a larger shear stress. Elasticity of typical cements allows “acceptable migration” due to the application of force, but when a cement's coefficient of elasticity is reached, the cement will fail with significant loosening, requiring a revision. Other factors that can contribute to cement failure are:                1. Variance in quality of manufactured cement—Quality control        2. Variance in bone composition from one individual to another, which could mean a difference in adhesion        3. Variance in operation technique.        4. Too much or too little cement applied.        5. Foreign material getting into the cement.        6. Different levels of patient activity may exert significantly higher amounts of force on the shell, causing varying degrees of migration.        